var banner = new Array();

banner.push('https://i.servimg.com/u/f65/16/15/57/49/bannia10.jpg');
banner.push('https://i.servimg.com/u/f65/16/15/57/49/bannia10.png');
banner.push('https://i.servimg.com/u/f65/16/15/57/49/bannia11.png');
banner.push('https://i.servimg.com/u/f65/16/15/57/49/bannia12.png');

document.getElementById('logo').firstChild.src= banner[Math.floor(Math.random()*banner.length)];